1. Field of the Invention
The present invention relates to a rotary magnetic head apparatus for recording signals on a magnetic tape as a recording medium and for reproducing signals recorded on the magnetic tape, and more particularly to a rotary magnetic head apparatus that can suitably be applied to a digital audio taperecorder or DAT.
2. Prior Art
At the initial stage of development, the digital audio taperecorders were close in size to 8-mm video taperecorders (VTRs), so that they used the 8-mm VTR specifications as is.
FIG. 1 shows a cross section of one example of a rotary magnetic head apparatus used on the DAT. In the figure, reference numeral 1 represents a chassis, and 2 a stationary drum mounted on the chassis 1. At the top of the stationary drum 2 is formed an annular recessed portion 2a. Motor 3 is mounted to the underside of the stationary drum 2. A motor shaft 3a passes through and projects above the stationary drum 2. Rotary drum 4 is secured to the shaft 3a that projects from the stationary drum 2. A magnetic head 5 is mounted on a mounting base 6, which is secured to the underside of the rotary drum 4, in such a way that the outer end of the magnetic head 5 slightly projects outwardly from the outer circumferential surfaces of the stationary drum 2 and of the rotary drum 4.
The magnetic head 5 has a coil, though not shown, connected to a primary winding 7a or 7b of a rotary transformer described later. Core 7 is located on the rotor side that forms a part of a rotary transformer and which is attached to the underside of the rotary drum 4 and provided with the primary windings 7a, 7b connected to the coil of the magnetic head 5. Core 8 is located on the stator side that forms a part of the rotary transformer and which is installed at the bottom of the recessed portion 2a of the stationary drum 2 in such a manner that it faces the rotor side core 7 with a gap formed therbetween (e.g. 30 to 50 .mu.m) therebetween small enough to enhance a coupling coefficient between rotor side core 7 and stator side core 8. The stator side 8 is provided with secondary windings 8a, 8b at positions opposite to the primary windings 7a, 7b. T is a magnetic tape as a recording medium.
FIG. 2 shows a transfer characteristic of the rotary transformer in a frequency band in which it is used. In the figure, a represents a step-up ratio equal to the number of turns n.sub.S of the secondary windings 8a, 8b divided by the number of turns n.sub.P of the primary windings 7a, 7b; E.sub.O is an electromotive voltage; E.sub.i is an input voltage of a reproducing amplifier; and f.sub.r is a resonance frequency.
Now, the operation of the DAT will be explained.
As the motor 3 is started to rotate the rotary drum 4 mounted on the shaft 3a and the magnetic head 5 secured to the rotary drum 4 through the mounting base 6 is also rotated. With the rotary drum 4 and the magnetic head 5 rotating, the magnetic tape T is moved by a tape transport mechanism to record signals on or reproduce them from the magnetic tape T by the magnetic head 5.
The rotary transformer is used to supply signals, which are to be recorded on the magnetic tape T, from a recording amplifier to the magnetic head 5, and also to supply signals reproduced from the magnetic tape T by the magnetic head 5 to a reproduction or playback amplifier not shown.
Next, we will deal with the number of turns n.sub.H of coil on the magnetic head 5, the number of turns n.sub.P of the primary windings 7a, 7b on the rotary transformer, and the number of turns n.sub.S of the secondary windings 8a, 8b on the rotary transformer.
The rotary transformer is required to have as small a loss and as flat a transfer characteristic as possible in the service frequency band as shown by the solid line in FIG. 2. It is a general practice with VTR that the frequency of the white peak of a video signal is set to coincide with the resonance frequency f.sub.r.
Thus, if we let the input capacity of the playback amplifier C, then the resonance frequency f.sub.r is expressed as ##EQU1## where L.sub.SH is an inductance of the rotary transformer impedance as seen from the secondary windings 8a, 8b.
Then, the inductance L.sub.SH is given by ##EQU2## where L.sub.P is an inductance of the primary windings 7a, 7b of the rotary transformer; L.sub.H is an inductance of the coil of the magnetic head 5; and K is a coupling coefficient of the rotary transformer (usually 0.97 to 0.99; the greater the better).
Since the resonance frequency f.sub.r and the input capacity C are already known, the inductance L.sub.SH can now be determined.
Because the transfer characteristic of the rotary transformer is given by ##EQU3## Then the inductance L.sub.P that makes E.sub.i /E.sub.O maximum is given by ##EQU4## Now, the relationship between the two inductances L.sub.P and L.sub.H can be obtained.
The optimum step-up ratio a is expressed as ##EQU5##
The number of turns of coil n.sub.H, the number of turns of primary winding n.sub.P and the number of turns of secondary winding n.sub.S are determined so that they meet the above relationships. In general, n.sub.H is set at 20 to 25; n.sub.P at 3 to 5; n.sub.S at 6 to 15; and the step-up ratio a at 2 to 3.
The conventional rotary magnetic head apparatus is constructed as mentioned above. With the number of turns n.sub.H, n.sub.P, n.sub.S set as described above, the number of turns of coil n.sub.H for the magnetic head 5 should be in the range of 20 to 25. This means that it is necessary to pass the wire through a narrow gap as small as a needle hole repetitively 20 to 25 times. This coiling process is carried out either manually or by using an automatic wire winding machine.
It will generally take about three months before a workman is reasonably proficient at winding the wire to form 20 to 25 turns. When the coil is to be made by the automatic wire winding machine, a costly, high precision facility is required. The overall cost of the facility therefore is higher than that of manual work. This in turn raises the cost of magnetic head 5 and therefore the rotary magnetic head apparatus.